It is widely known that magnetic resonance imaging (MRI) is capable of yielding useable information about the composition of internal biological tissues. One very important and widely used application of MRI is for noninvasive clinical imaging of internal portions of the human body. As is well known, MRI relies on the nuclear magnetic resonance of nuclei to produce maps or images of those internal portions of the human body. Importantly, MRI is a non-invasive procedure which can be safely and efficaciously used to acquire medical information about specific internal tissues.
It is known that certain atomic nuclei have nuclear magnetic moments which, when placed in a static magnetic field can only take up certain discrete orientations. Each of these orientations corresponds to a different energy state for the nucleus. Further, it is known that the application of radio frequency (RF) electromagnetic radiation, also referred to as electromagnetic waves, to nuclei in a magnetic field can induce a transition in the energy state of nuclei from one level to another. Such a transition is known as nuclear magnetic resonance (NMR).
A particular frequency of RF electromagnetic waves, known as the Larmor frequency, is the most effective frequency for inducing a change in the energy state and the corresponding orientation of the magnetic moment of a particular nucleus in a magnetic field. Specifically, the Larmor frequency for each nucleus is proportional to the magnitude of the magnetic field at the location of the particular nucleus. To image nuclei, electromagnetic waves at the Larmor frequency for the nuclei to be imaged are transmitted by an antenna that is connected to a transmitter. The electromagnetic waves transmitted at the Larmor frequencies induce changes in the orientations of the magnetic moments of the nuclei to be imaged. Subsequently, as the magnetic moments of the nuclei return to their initial orientations, the magnetic moments of the nuclei generate detectable electromagnetic waves, which can be refocused to form spin echoes. The characteristics of these spin echoes are representative of the local environment of the nuclei being imaged. The spin echoes generated by the nuclei are detected by an RF receiver. Importantly, the detected electromagnetic waves oscillate at the local Larmor frequency. From the foregoing discussion, it is appreciated that nuclei placed within a range of magnetic fields will oscillate within a corresponding range of Larmor frequencies.
Most MRI devices utilize a static magnetic field that is homogeneous. In a homogeneous magnetic field, the components of the gradient of the magnitude of the magnetic field are all substantially equal to zero, (G.sub.X =G.sub.Y =G.sub.Z =0) at all points within the imaging region of the field. Hence the oscillating RF bandwidth is relatively narrow. In contrast, in a nonhomogeneous magnetic field at least one of the components of a gradient, G.sub.Z, is not equal to zero. U.S. Pat. No. 4,498,048, which issued to Lee et al., for a device entitled "NMR Imaging Apparatus" is an example of an MRI device which has a substantially homogeneous magnetic field. In contrast, U.S. Pat. No. 5,304,930 (hereinafter the '930 patent), which issued to Crowley et al., for a device entitled "Remotely Positioned MRI System" is an example of an MRI device which has a non-homogeneous static magnetic field. Importantly, the '930 patent also discloses methods for producing images with an MRI device having a substantially non-zero magnetic field gradient.
For both MRI devices that have a homogeneous magnetic field and MRI devices that have a non-homogeneous magnetic field, it is important to shape and position the magnets to provide adequate access to the magnetic field for the patient. Providing for comfortable placement of the patient while providing adequate access to the magnetic field region is important to producing high quality medical images. Large magnets with their corresponding support structure can hinder patient access to the magnetic field of an MRI device. For MRI devices that have a homogeneous magnetic field, the physical dimensions of the magnet are substantially larger than the imaging volume needed to make medically useful images of internal portions of a human body. Due to the bulk of the magnets in MRI devices that have a homogeneous magnetic field, the size of the opening for patient access to the magnetic field in these devices is generally restricted in size. This limited access results from the fact that producing a high degree of field homogeneity requires a careful juxtaposition of magnetic field sources. By design, the juxtaposition of these sources is such that the field of these gradients mutually cancel to a high degree of precision to give the result G.sub.x =G.sub.y =G.sub.z =0. Magnetic sources must often be placed throughout the periphery of the imaging volume in order to achieve such mutual cancellation, thereby restricting peripheral access. In contrast MRI devices that have a nonhomogeneous magnetic field do not require complete cancellation of the static field gradients. As a result, some peripheral portion of the magnet may be left open, thereby providing less restrictive patient access. In addition to providing patient access, a non-uniform field magnet is easier to construct, and is less sensitive to variations in temperature and engineering tolerances.
The magnet system disclosed in the '930 patent is an example representative of this general trend towards less restrictive access magnets. In particular, a magnet having a non-uniform magnetic field neither surrounds nor confines the tissue being imaged. Instead, the magnet is juxtaposed against the tissue to be imaged. The present invention recognizes that there are compelling reasons to use magnets that may be somewhat more confining than a remotely positioned magnet while still retaining the benefits of peripheral access in a nonhomogeneous magnetic field. In particular, the present invention recognizes that some juxtaposition of magnet sources may be used to reduce the relative magnitude of the aforementioned non-zero field gradient while still providing substantial access to the imaging region within the magnet. As such, the structure of the present invention can be conceptually considered to be between an open, remotely positioned magnet and a closed homogeneous field magnet.
There are several benefits to this approach and they all derive from the relationship between the range of magnetic field values in the static field gradient. In particular, if the nonhomogeneous field gradient is reduced, then the corresponding range of Larmor frequencies is reduced over the specified image volume. From the standpoint of the RF transmittal cited previously, this typically reduces the peak power requirements by reducing the required bandwidth. Reducing RF transmitter power reduces undesirable RF exposure to the patient, energy consumption of the electronic circuitry, and the complexity and size of the transmitter and corresponding power supplies. From the standpoint of the RF receiver, the correspondingly reduced bandwidth for received signals is accompanied by a reduced bandwidth of thermal noise and the signal-to-noise ratio (SNR) is correspondingly increased.
As recognized by the present invention, an increased SNR, and a decrease in the amount of RF transmitter power required to transmit RF waves at Larmor frequencies can be realized by properly shaping the static magnetic field. The magnetic field is shaped by juxtapositioning magnetic field sources to reduce the magnitude of the magnetic field gradient G.sub.z without attempting to completely cancel that gradient. Reducing the magnitude of the magnetic field gradient, G.sub.z, decreases the range of the magnitude of the magnetic field over a fixed imaging volume, thereby reducing the bandwidth of the Larmor frequencies required to image that volume. The reduced bandwidth of Larmor frequencies permits reducing the bandwidth of the RF receiver, which results in reception of less noise power by the receiver and a corresponding increase in the SNR. Reducing the bandwidth of the Larmor frequencies also permits reducing the bandwidth of the RF transmitter, which correspondingly reduces transmitter power.
In light of the above, it is an object of the present invention to provide an MRI apparatus for imaging an object, with open access to the magnetic field. It is another object of the present invention to provide an MRI apparatus with a small but substantially non-zero gradient of the magnetic field magnitude to produce a relatively larger SNR and to realize relatively smaller RF transmitter power. Yet another object of the present invention is to provide an MRI apparatus for imaging an object which is easy to implement, simple to use, portable, and comparatively cost effective.